NEW YORK (CNNMoney.com) -- Government regulators said Friday they are analyzing recent study results regarding cholesterol drug Vytorin, but that it's too early to tell whether it will take any regulatory action.

The Food and Drug Administration is focusing on study results from drugmakers Merck (MRK, Fortune 500). and Schering-Plough (SGP, Fortune 500), who unveiled their study on Jan. 14. The study failed to prove that Vytorin, a combination drug containing Schering's Zetia and Merck's generic drug Zocor, is better than Zocor alone in reducing plaque in neck arteries.

But Vytorin is still seen as effective in lowering harmful types of cholesterol, which is the drug's FDA-approved purpose.

Vytorin is a $4 billion-a-year treatment, with sales split between Schering and Merck. Vytorin is a name-brand drug, and triple the cost of generic Zocor.

Since the study was released, the stocks for each of these companies have dropped more than 15 percent. Merck and Schering have since launched an ad campaign, saying that they "proudly stand behind the established efficacy and safety profiles of Zetia and Vytorin."

The FDA said there is no reason to believe that the drug has safety issues, beyond those that are already identified in the label.

"Our main focus is going to be on any safety concerns," said John Jenkins, director of the FDA's Office for New Drugs, in a teleconference with reporters. But he also said the FDA has "not identified any safety issue per se, above and beyond what's in the labeling."

Les Funtleyder, analyst for Miller Tabak, said the FDA was conducting its analysis primarily because of heightened concerns in the post-Vioxx era, a reference to Merck's anti-inflammatory drug that was withdrawn in 2004 because of heart attack and stroke risks. This action inspired tens of thousands of lawsuits against Merck and undermined the FDA's credibility.

"I think in the shadow of Vioxx, the FDA has a higher sensitivity level to public concerns - which are driven by the media," said Funtleyder, in an email. "If there is an actual safety issue with the drug, it's better for them to be proactive than reactive."

The analyst said the FDA is also concerned that people will stop taking cholesterol-lowering drugs, which feeds the need for further study.